“I’m thrilled that they have this giant IPO,” said Rick Marini, founder and CEO of BranchOut, another professional networking service. “It brings visibility and credibility to what we’re doing.” The not-quite year-old company earlier this month closed an $18 million round of fund raising led by Redpoint Ventures.

That is money BranchOut plans to use to at least quadruple its sales staff of just three people. Marini naturally thinks BranchOut’s business is sounder than LinkedIn, which is decidedly bigger — and has a nine-year head start.

Unlike LinkedIn, which started a professional network from scratch, Marini is building his business on the back of Facebook, the future IPO darling that has more than 600 million users. BranchOut figures people already are mixing work and play on Facebook, and it is a natural place for recruiters to find new hires, and for sales people to connect with prospective business contacts.

Marini said BranchOut’s college intern last year “had never heard of LinkedIn but had 1,400 friends on Facebook.”

Of course, having a business tied so closely to Facebook can be a risk, too. “When people say are you worried being beholden to another platform, my answer is one word: Zynga,” said Marini. The company behind diverting Facebook games such as FarmVille recently was in fund-raising talks that valued the company at roughly $10 billion, The Wall Street Journal reported.

Marini said he was surprised to see LinkedIn’s IPO price creep up from $35 a share to $45 to as high as $122.70 today before closing at $94.25. That values the public-market newbie LinkedIn at $8.9 billion — a good sign for other social media companies. And Marini knows it.

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